Abstract

Sediments deposited in deltas may contain commercially important amounts of gas, high pour-point crude oils and crudes with normal pour points. The oil and gas have been generated from the organic matter incorporated into these sediments during deposition. The nature of the organic matter exercises initial control over the type of hydrocarbons generated, with algal material yielding normal crudes, woody material giving gas, and waxy crudes being generated from land-plant coatings. The distribution of oil, waxy oil, and gas reflects the initial distribution of organic matter types and gives an ideal lateral sequence from gas nearshore to oil offshore and a vertical sequence from gas shallow to oil deep. These distributions may be disturbed by subsequent movements induced tectonically. In particular, differential entrapment during secondary migration leads to a preferential movement of oil updip. This is generally toward the paleoshoreline in Tertiary delta systems, but in older deltas the direction of movement is more variable. The character of the oil can be changed considerably by biodegradation in the reservoir and waxy crudes may be converted to lower pour-point oils with lower API gravities and higher sulfur contents. These effects can be augmented by water washing. Thermal effects also lead to dramatic changes in crude oils making them steadily lighter until ultimately they are converted to gas. Thus, the distribution of oil and gas in deltas is controlled originally by the distribution of organic matter types, but is later modified by the effects of secondary migration, maturation, and alteration.